Since the discovery by D. Kirkwood in 1866, the orbits in
the main asteroid belt with periods commensurable with that
of Jupiter have been recognized as locations where no
asteroids are found and thereby called "gaps". It is widely
believed that asteroid gaps were formed by resonant and
chaotic actions caused by the gravity of Jupiter, and they
are regarded as a supply source of near-Earth asteroids
(NEAs), the majority of which are sub-km-sized. Here we
report that, for asteroids smaller than ~1km in
diameter, the 3:1 gap (a=2.50AU) substantially does not
exist and is almost filled with asteroids. Assuming that the
asteroid population inside the gap is in steady state, a
distribution as such was constructed from 2 x 104 yr
orbital integration of about 5200 existing asteroids near
the gap. From the steady-state relative number distribution
of asteroids in the gap and the size distribution for
km-sized asteroids outside the gap edge which was recently
observed by our group, we found that the slope (~
2.7-3.1) for the cumulative size distribution of km-sized
asteroids in the gap is much steeper than that for the
corresponding NEAs in size (~1.8: Bottke et al., 2002).
This inconsistency and very short dynamical lifetimes (a few
Myr) of gap asteroids suggest that the 3:1 resonant gap may
not be such a dominant source of NEAs as previously claimed,
though it will still be an important one. We found a similar
gap-filling effect also in the 7:3 Kirkwood gap
(a=2.96AU).

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